The rest of the airport remained open, but a number of flights were delayed.

The powder - which the man was carrying in both his hand luggage and hold baggage - was later found to be harmless.

An Explosive Ordinance Device Team attended the scene but found no evidence the powder could be used as a detonator.

The passenger helped detectives with their inquiries at the scene as the tests were carried out but no arrests were made in connection with the incident.

Superintendent Leor Giladi said: 'Because the powder was unidentified we had to take every precaution to protect the safety of passengers at the airport.

'Public safety is our primary concern and that is why we had to take the action we did.

'Tests have been carried out on the substance that have concluded it could not have been used for any explosive purposes and there was absolutely no risk to anyone at the airport.

'I would like to take this opportunity to thank all those passengers who may have experienced some delays for their patience.'

Bus bomb: The attack in Tavistock Square, London, on July 7 2005

The passenger had been trying to board flight BD589 to Heathrow, which was due to depart at 1.55pm.

It is understood he told airport staff that he was carrying a 'freeze-dried compound' on behalf of a friend.

Passengers were having to check in at Terminal 1 last night before being bussed back to Terminal 3 in order to depart.

It is thought the Home Office's decision to increase the terror alert level on Friday night was prompted by the impending visit by Hilary Clinton.

Professor Richard Bonney believes the arrival of the U.S. Secretary of State in London next week is why the official level has gone from substantial to severe.

Home Secretary Alan Johnson announced the shift last night. The new level denotes that an attack is 'highly likely' and is the second highest on a five-step scale.

Warning: The Home Office increased the terror alert level last night ahead of Hilary Clinton's visit to London

Mrs Clinton and other leading foreign ministers are due in London this Wednesday and Thursday for two major summits on Afghanistan and Yemen.

Also at the meetings will be Afghan leader Hamid Karzai and the UN Secretary General Ban-Ki Moon.

Professor Bonney, from the University of Leicester said: 'I suppose that Hilary Clinton's arrival is the precipitant; but the
Afghan conference is a high profile target, given the number of
interested players. (Her visit to London) is a very likely, plausible threat.

'It's a high profile target for some groups, those who seek to disrupt
any sort of future peace process in Afghanistan in particular.'

Intelligence sources said the decision followed a 'non-specific'
warning from the CIA that the UK is facing an increased threat from Al
Qaeda.

The terror threat has not been at this level since July last year.

Mr Johnson insisted there was no intelligence to suggest an attack is imminent, but said the public should be 'more aware'.

Prof Bonney said unless there was some other associated threat that
intelligence agencies had been made aware of, it was likely the level will drop back to 'substantial' shortly after the conference.

'It may be maintained for a period afterwards but unless there's
some other associated threat which we don't know about it's unlikely it
would continue for more than a month,' he said.

Prof Bonney said it was unlikely Al Qaeda would be considering an
attack, but affiliated groups may be interested in the conference as a
target.

'Increasingly we're seeing the fragmentation of groups, and this is why
security forces are having difficult controlling the situation and why
resources are fully stretched.'

The move underlines the continuing threat to Britain and increased fears around the globe after the failed Christmas Day attack on a plane over Detroit.

The would-be bomber, Umar Farouk Abdulmutallab, was a student in London
before apparently going to Yemen for terror training by Al Qaeda.

Abdulmutallab, who was manhandled to the floor of the jet by passengers after trying to blow up explosives hidden in his underpants as the jet came in to land, warned after his arrest that there were 'many more like me' plotting to strike.

Sources reportedly said the Detroit plot was 'one of the factors' behind raising the alert level but that the move was taken after looking at intelligence 'in the round'.

It is thought intelligence agencies working round the clock in the UK to pick up any possible terror plans had spotted an upsurge in 'chatter' on the internet, which is a key indicator of a possible attack.

The UK's official terror threat was at the severe level for four years after the July 7 bombings in London in 2005 but was downgraded last summer.

Mr Johnson said last night: 'The Joint Terrorism Analysis Centre has
today raised the threat to the UK from international terrorism from
substantial to severe.

'This means that a terrorist attack is
highly likely, but I should stress that there is no intelligence to
suggest that an attack is imminent.

'JTAC keeps the threat level
under constant review and makes its judgments based on a broad range of
factors, including the intent and capabilities of international
terrorist groups in the UK and overseas.'

He added: 'We still
face a real and serious threat from international terrorism, so I would
urge the public to remain vigilant and carry on reporting suspicious
events to the appropriate authorities and to support the police and
security services in their continuing efforts to discover, track and
disrupt terrorist activity.'

Held: Umar Farouk Abdulmutallab

A spokesman from the Metropolitan Police said the service would review its contingency planning and policing tactics following the announcement.

'People who live, work and visit the capital will continue to see a wide selection of policing tactics being used,' he said.

Raising the terror threat level is mainly designed to increase the vigilance of the emergency services and those working in security jobs, another terrorism expert said.

'I think it's not something that the public will notice very much, in terms of everyday life, said Professor Paul Wilkinson, of the University of St Andrews.

'The level of preparedness and vigilance will be increased. The whole idea is to give the various services, including the emergency services, an update on the threat level.

'It is mainly designed for the emergency services, including the police, and people who work in security jobs, for example in airports and sea ports.'

Professor Wilkinson said: 'People should remember that we are not dealing with an old fashioned national terrorist or separatist group, but a form of transnational terrorism, a network which has affiliates in 90 countries - that's half the countries in the world.'

He said the fact that the level was not raised after the Christmas incident indicated that there is not simply a knee jerk reaction.

'They try to examine all the evidence they have about changing trends. We must remember that the security services have been tremendously successful in preventing some of the worst conspiracies.'

Last week, in a statement on the Detroit incident, Gordon Brown told MPs that a number of terror cells were 'actively trying to attack Britain'.

He announced the creation of a new no-fly list of terror suspects, enhanced security screening at airports and the suspension of flights to Yemen.

The independent reviewer of anti-terrorism legislation, Lord Carlile of Berriew, said the Government was right to announce the change in the threat level.

'It is absolutely essential that there should be public vigilance and the Government has - quite rightly - decided that if you don't tell the public to be vigilant, they are not going to be vigilant,' Lord Carlile told the BBC.

'The message from the current change of assessment is not that we should be more afraid but that we should be a little bit more vigilant than we have been.'

Tory MP Patrick Mercer, chairman of the Commons counter-terrorism sub-committee, said last night it was not surprising the terror threat had been upgraded.

He added: 'The reduced level we have had recently is unusual - we are back to the norm'.